Cerium-based abrasive material and method for preparation thereof

ABSTRACT

A cerium-based abrasive and a production method of the cerium-based abrasive excellent in polishing properties of a high polishing speed and scarce formation of polishing scratches are provided by keeping the color of the abrasive in specified ranges or stably making fluorine be contained in the abrasive. For example, as such a cerium-based abrasive, examples include a cerium-based abrasive containing cerium oxide as a main component and having an L* value in a range not lower than 65 and or lower 90, an a* value in a range 0 or higher but 15 or lower, and a b* value in a range 10 or higher but 30 or lower in the case the color is expressed by an L*a*b* color system.

CROSS REFERENCE TO RELATED APPLICATION

This application is a 35 USC § 371 National Phase Entry Application fromPCT/JP02100762, filed Jan. 31, 2002, and designating the U.S.

TECHNICAL FIELD

The present invention relates to a cerium-based abrasive and aproduction method of the cerium-based abrasive, more particularly, acerium-based abrasive and a production method of the cerium-basedabrasive excellent in polishing properties such as polishing speed-andpolished surface.

BACKGROUND ART

Recently, an abrasive is employed for a variety of purposes. Above all,highly precise surface polishing is required for glass substrates suchas a glass substrate for an optical disk, a glass substrate for amagnetic disk, a glass substrate for a photomask to be employed forsemiconductor fabrication, a glass substrate for LCD and the like, anoptical lens and the like.

For surface polishing for them, rare earth oxides, especially, acerium-based abrasive containing cerium oxide as a mainly component(hereinafter referred as to a cerium-based abrasive) has been employed.Because as compared with zirconium oxide and silicon dioxide, ceriumoxide is excellent in polishing properties for polishing glass. Forexample, since it has a high polishing speed as compared with zirconiumoxide, silicon dioxide, aluminium oxide, and the like, cerium oxide iscapable of performing polishing faster. Further, since cerium oxide hasa hardness not so high, smoother glass surface can be obtained afterpolishing by using cerium oxide for polishing.

However, the polishing properties of the cerium-based abrasive aredetermined depending on a large number of factors such as contents ofcerium oxide, fluorine (F) and the like, a specific surface area and thelike. Consequently, to control the polishing properties, there areproblems that the respective properties have to be precisely evaluatedand comprehensively judged.

Further, the production conditions of the cerium-based abrasive are alsoimportant. For example, as one of most important steps, there is aroasting step in the latter half of the production process. The roastingstep is a step of heating raw materials for the cerium-based abrasive ata high temperature in an oxidative atmosphere and in the step, fluorine,one of important factors determining the polishing properties, is easyto be dissipated from raw materials. Consequently, the roasting step hasto be properly controlled. Especially, in the case of using an apparatuscapable of carrying out continuous roasting, it is difficult to quicklyand precisely judge the roasting state, thereby leaving difficulty incontrol as a problem.

The invention is developed to deal with such problems and aims toprovide a cerium-based abrasive and a production method of thecerium-based abrasive excellent in polishing properties.

DISCLOSURE OF THE INVENTION

In polishing using a cerium-based abrasive, the following are required:the polishing speed is high (the polishing efficiency is high) and atthe same time no scratch is formed and further the remaining abrasiveafter polishing is excellent to be cleaned out. In order to achieve suchpurposes, various physical values such as the size of the abrasiveparticles and chemical compositions have been investigated and variouslytried to be optimized. No adhering to the viewpoints of the propertiesof an abrasive which have been thought ever before, inventors of theinvention have made various investigations regarding abrasives evaluatedto be excellent in polishing evaluation. As a result, it has been foundthe color of a cerium-based abrasive and the polishing evaluation have acorrelation. Further investigations have consequently made it clear thatan excellent abrasive can be obtained by optimizing the color of thecerium-based abrasive to complete the invention.

In a cerium-based abrasive containing cerium oxide as a main component,the invention has a characteristic that the cerium-based abrasive has anL* value 65 or higher and 90 or lower when a color of the cerium-basedabrasive is expressed in an L*a*b* color system.

The color system L*a*b* (L-star, a-star, b-star) defined in JIS Z 8729is very frequently used in the industrial field for color management anda color is expressed by a value of L*, a value of a*, and a value of b*(hereinafter referred as to L* value, a* value, and b* value,respectively). L* expresses brightness and is also called as “brightnessindex”. On the other hand, a* and b* express hue and chromaticness andare also called as “chromaticness index”. In the L*a*b* color system, asan L* value increases, a color becomes closer to white and as decreases,closer to black. Also, as an a* value is increased more in the plusside, a red type color is intensified more and as decreased more(increased more in the minus side), a green type color is intensified.Also, as a b* value is increased more in the plus side, a yellow typecolor is intensified more and as decreased more (increased more in theminus side), a blue type color is intensified. Also, if both a* valueand b* value are zero together, it means colorless.

As a result of investigations, it has been found that, for acerium-based abrasive, the state with a low L* value is a stateparticles of the abrasive are excessively grown and also a state withmany coarse particles causing scratch formation at the time ofpolishing. If an abrasive in such a state is used, polishing scratchesare easy to be formed, resulting in impossibility of obtaining asmoothly polished surface. On the other hand, it has also been foundthat the state with a high L* value is a state the particle growth byroasting is insufficiently promoted, resulting in impossibility ofobtaining a sufficiently high polishing speed, difficulty of evendissipation in a dispersant, and easiness of scratch formation owing toremaining huge agglomerated particles. As a result of suchinvestigations, it is found that a cerium-based abrasive with an L*value of 65 or higher and 90 or lower, preferably 70 or higher and 80 orlower, hardly causes scratches and is excellent in polishing propertiessuch as smoothness.

The a* value in the L*a*b* color system is preferably 0 or higher and 15or lower.

That is because if the a* value is lower than the range, the content offluorine is so low as to make it impossible to cause chemical reactionsneeded at the time of polishing glass and make the extremely small roughstate of the surface to be polished smooth. On the other hand, that isalso because if the a* value is higher than the range, yet the polishingeffect is high, the abrasive excessively contains fluorine and it is notpreferable. Further, the content of the coarse particles is high toresult in easiness of polishing scratch formation. That is supposedlyattributed to the particle growth was excess at the time of roasting.From such viewpoints, the a* value is preferably 5 or higher and 15 orlower.

The b* value in the L*a*b* color system is preferably 10 or higher and30 or lower.

That is because if the b* value is lower than the range, the contents offluorine and praseodymium oxide are so low as to make it impossible tocause chemical reactions needed at the time of polishing glass and makethe extremely small rough state of the surface to be polished smooth. Onthe other hand, that is also because if the b* value is higher than therange, yet the polishing effect (the polishing efficiency) is high, theabrasive excessively contains fluorine and is in a state that manycoarse particles to be a cause of the polishing scratches are formed andit is not preferable. That is supposedly attributed to the particlegrowth was excess at the time of roasting. From such viewpoints, the b*value is preferably 20 or higher but 25 or lower.

As it is made understandable from above descriptions, a cerium-basedabrasive with an L* value of the color system L*a*b* 65 or higher and 90or lower, an a* value 0 or higher and 15 or lower, and a b* value 10 orhigher but 30 or lower has a sufficiently high polishing speed, scarcelycauses polishing scratches, scarcely leaves a remaining abrasive, and isstably provided with these excellent polishing properties. Consequently,such an abrasive is suitable to be used for precision polishing forwhich surface smoothness after polishing is especially required. It isfurther preferable if the L* value is 70 or higher but 80 or lower, thea* value is 5 or higher but 15 or lower, and the b* value is 20 orhigher but 25 or lower. Incidentally, the L* value, the a* value, andthe b* value can be recognized with eye observation by a person or ameasurement apparatus. Consequently, for example, the quality can bejudged or controlled by specifying the color of an abrasive by comparingthe color of the cerium-based abrasive with that of a standardizedproduct or the quality can be judged or controlled by specifying thecolor by employing a measurement apparatus.

Incidentally, the color control to make a cerium-based abrasive have adesired color can be actualized by controlling the content of fluorineand further controlling the content of praseodymium oxide. For example,the color of an abrasive powder is measured in the roasting step in theproduction process and the roasting temperature, the roasting duration,the gas circulation state in a roasting furnace, and the supply speed tothe roasting furnace are adjusted to easily keep the L* value, the a*value, and the b* value in specified ranges, respectively, in the L*a*b*color system. The polishing properties regarding the polishing speed,the polishing scratches, the remaining abrasive and the like can easilybe controlled by keeping the color of the cerium-based abrasive in theabove-described specific ranges.

The cerium-based abrasive contains, as rare earth oxides other thancerium oxide (CeO₂), lanthanum oxide (La₂O₃), neodymium oxide (Nd₂O₃),praseodymium oxide (Pr₆O₁₁), samarium oxide (Sm₂O₃), and the like. Sincerare earth elements are similar in chemical and physical properties toone another, they are difficult to be separated. However, they do notsignificantly inhibit the effect as an abrasive. Those to be used asabrasives are those containing cerium oxide as a main component andgenerally, the ratio, that is, the content, of cerium oxide in the totalrare earth oxides (hereinafter, abbreviated as TREO) is required to be30% by weight or higher. Because if the content of cerium oxide in TREOis less than 30% by weight, a sufficient polishing speed cannot beobtained to result in insufficiency for practical use. If the value is30% by weight or higher, practical use for polishing, especially forpolishing glass is possible.

Further, in the cerium-based abrasive, other than rare earth oxides,there remain oxides of such as Si, Al, Na, K, Ca, Ba and the like orcompounds other than oxides contained in minerals such as bastnasite.Some of them have a function as an abrasive, yet the function is low,and become a cause of polishing scratches. For that, the content of TREOin the cerium-based abrasive is controlled to be 80% by weight or higherbut 99% by weight or lower. If TREO is less than 80% by weight, manypolishing scratches are formed. If TREO exceeds 99% by weight, theproductivity is lowered in order to increase the purity.

Further, it is important to contain a fluorine component as thecerium-based abrasive, especially as an abrasive for glass. That is,because it is supposed to be probable that existence of the fluorinecomponent makes chemical polishing possible to increase the smoothnessof the polishing surface.

As a result of such investigations, the cerium-based abrasive of theinvention is found preferable to have the content of TREO in itself 80%by weight or higher but 99% by weight or lower and the content offluorine in TREO 0.5% by weight or higher but 10% by weight or lower.

Because it is found if the content of fluorine in TREO is 0.5% by weightor higher, an excellent polishing effect especially for use forpolishing glass can be obtained owing to not only simply physicalpolishing but also chemical reactions by fluorine. Because, in otherwords, if the content of fluorine is less than 0.5% by weight, thepolishing speed is too low for practical use. Further if the content offluorine exceeds 10% by weight, an interior of a roasting furnace isvulnerable to corrosion and the cost for waste-gas treatment will behigher, disadvantageously.

Incidentally, the content of fluorine is a value calculated byquantitatively measuring fluorine of the same specimen as a specimen ofwhich TREO is measured and calculating the weight as the content inTREO. The reason for doing that is because almost all of fluorine islost at the time of TREO measurement and therefore even if fluorine ismeasured in a specimen obtained after the TREO measurement, no precisevalue can be obtained.

The cerium-based abrasive is preferable to have a content ofpraseodymium oxide contained in TREO 1% by weight or higher but 10% byweight or lower.

As described above, it is important for the cerium-based abrasive,especially as an abrasive for glass, to contain a fluorine component.Consequently, if dissipation of fluorine at the time of roastingproceeds to the extent too far, the probability that the fluorinecomponent contained in the abrasive is too slight is high and it is notpreferable. An investigation of the roasting step has, therefore, beenmade to find difference exists in the degree of the dissipation of thefluorine component even if the roasting conditions are made constant inthe roasting step.

Hence, the chemical behavior of rare earth elements other than cerium,and fluorine at the time of roasting has been investigated. As a result,a variety of light rare earth elements other than cerium are foundhaving the effectiveness of the effect to prevent the dissipation offluorine in the roasting step and among them, especially, existence ofpraseodymium or praseodymium oxide is excellent in thedissipation-preventive effect on the fluorine. For example, in the casefluorine is immobilized in form of a fluorine-containing compound ofpraseodymium, a chemical polishing function in polishing, supposedlyderived from the fluorine component, can be obtained and consequently, ahigh polishing speed can be obtained and excellent surface smoothness isobtained.

The reasons for the excellent dissipation-preventive effect aresupposedly as follows. At first, praseodymium in a form of praseodymiumcompounds (e.g., praseodymium oxide), the oxidation number can be notonly +3, which is common in the case of rare earth elements, but also avalue higher than +3, and especially a value higher than +3 is stable inthe case of praseodymium oxide: praseodymium forms a compositecomposition of the oxide and the fluoride and it is thermally stable:and the like.

From such investigation results, praseodymium compounds includingpraseodymium oxide were found preventing the dissipation of fluorine atthe time of roasting. And the particle growth of raw materials for thecerium-based abrasive is found possible to be controlled by adjustingthe content of praseodymium oxide to be a proper ratio. Hence, thecontent of fluorine and the content of praseodymium oxide necessary forpolishing have further been investigated.

As a result, the content of praseodymium oxide is found to bepreferably, as described above, 1% by weight or higher and 10% by weightor lower. Because if the content of praseodymium oxide is less than 1%by weight, the dissipation of fluorine at the time of roasting cannot beprevented. Further, in the case the content is low, the particle growthsometimes does not proceed and the obtained cerium-based abrasive doesnot have a sufficiently high polishing speed. Further, in order toadjust the content of praseodymium oxide to be lower than 1% by weightin the abrasive prediction steps, a special treatment is required toremove praseodymium compounds including praseodymium oxide to cause aproblem in terms of the cost. On the other hand, if the content ofpraseodymium oxide exceeds 10% by weight, immobilization of fluorine inthe roasting step is sufficient to obtain a sufficiently high polishingspeed, however there occurs a problem that the particle growth proceedsexcessively. Further, since the color of the cerium-based abrasivebecomes too dark owing to the excessively high content of praseodymiumoxide, the difference becomes difficult to be detected by colorimetricresults, especially based on the a* value to make quality control ofabrasives by colorimetry difficult. Further, as compared with ceriumoxide, praseodymium oxide has high hydrophilicity and in the case of anabrasive slurry, there occurs a problem that the polishing speed ischanged owing to the alteration of pH of the slurry.

Further, the cerium-based abrasive is preferable to have the specificsurface area of abrasive particles 1 m²/g or higher but 30 m²/g orlower.

If the specific surface area is lower than 1 m²/g, yet the polishingspeed is sufficiently high, there exists a problem that the polishingprecision is low and polishing scratches are many. If the specificsurface area exceeds 30 m²/g, if a slurry in which an abrasive issufficiently dispersed is used, a smoothly polished surface can beobtained after polishing, however preparation of a sufficientlydispersed slurry itself is difficult. Further, since a sufficientpolishing speed cannot be obtained, the work efficiency of polishingglass or the like is low to make practical use difficult. Further, sincethe amount of the remaining abrasive increases, it is a problem to useas an abrasive for precision polishing.

From such a point of view, if the value of the specific surface area iskept in the above-described range, an abrasive obtained is excellent inpolishing properties; a sufficiently high polishing speed, lessformation of polishing scratches at the time of polishing, capability ofgiving smoother polished face after polishing with little remaining ofthe abrasive on the polished face after polishing.

Incidentally, the cerium-based abrasive according to the invention maybe used as an abrasive slurry by being mixed with and dispersed in wateror an organic solvent. The concentration of the cerium-based abrasive(the solid matter) in the abrasive slurry is preferably 1% by weight orhigher but 40% by weight or lower, further preferably 5% by weight orhigher and 30% by weight or lower. Examples as the organic solventusable are alcohol, a polyhydric alcohol, tetrahydrofuran and the like.The reason for the setting of the cerium-based abrasive to be 1% byweight or higher but 40% by weight or lower is because if it is lowerthan 1% by weight, it causes undesirable results that the polishingefficiency is low owing to the low concentration and a large quantity ofpolishing waste solution is generated as well. On the other hand, it isalso because if it exceeds 40% by weight, the viscosity of the slurrybecomes high to make quantitative and constant supply of the slurrydifficult to result in uneven polishing and it is also not preferable.

The abrasive slurry may further contain additives such as a dispersingagent, a solidification-preventive agent, a pH adjusting agent and thelike. As the additives, examples are sodium hexametaphosphate, sodiumpyrophosphate, crystalline cellulose, calcium secondary phosphate,sodium β-naphthalene sulfonate-formalin condensate, synthetic silicondioxide, polyacrylic acid salt such as polyacrylic acid sodium salt,carboxymethyl cellulose, polyethylene oxide, polyvinyl alcohol and thelike. These additives may be mixed with the cerium-based abrasive atfirst and made to be slurry of the cerium-based abrasive containing theadditives; or the additives may be dissolved or dispersed at first inwater or an organic solvent and then the cerium-based abrasive isdispersed; or the additives may be added when the cerium-based abrasiveis made to be a slurry using water or an organic solvent. The weight ofthe additives is generally 0.1% by weight or higher but 4% by weight orlower to the weight of the cerium-based abrasive (the solid matter).That is because if less than 1% by weight, the effects of dissipation,solidification prevention, and pH adjustment are insufficient and ifexceeding 4% by weight, the effects are scarcely increased and on thecontrary, the effects are sometimes deteriorated.

Further, in order to solve the above-described problems, a method forproducing a cerium-based abrasive has been investigated. Thecerium-based abrasive is produced through the respective steps ofpulverizing raw materials of the cerium-based abrasive, carrying outtreatment with fluorine if necessary, roasting, pulverizing andclassifying the successively treated raw materials. Incidentally, awell-known treatment method to be carried out using hydrofluoric acid,ammonium fluoride and the like can optionally be employed for thetreatment with fluorine.

Inventors of the invention have made investigations while payingattention to the roasting step among these steps. The roasting step is astep of promoting the particle growth while carrying out sintering ofthe particles and also a step of dissipating fluorine at the same time.Consequently, if both of the particle growth and the fluorine contentcan be controlled in the roasting step, it is desirable for theproduction of the cerium-based abrasive excellent in polishingproperties. Therefore, a roasting step for controlling the dissipationof fluorine has been investigated.

As a result, it has been found that an abrasive can stably be sinteredand at the same time an amount of fluorine needed for the chemicalpolishing can reliably be maintained by controlling the fluorine contentafter roasting based on the fluorine content before roasting. That is,inventors of the invention have found it is possible to produce thecerium-based abrasive excellent in polishing properties and consequentlycapable of providing excellent polishing evaluations by measuring thecontent of fluorine before and after roasting and controlling thedecrease amount.

The invention provides a cerium-based abrasive production methodcomprising a step of roasting raw materials of the cerium-basedabrasive, characterized in that in the case the content of fluorine inrelation to TREO before roasting is defined as F1 (hereinafter simplyreferred as to F1) and the content of fluorine in relation to TREO afterroasting is defined as F2 (hereinafter simply referred as to F2), themethod keeps the ratio F2/F1 in a range 0.7 or higher but 1 or lower.

Because if F2/F1 is lower than 0.7, the particle growth proceedsexcessively to result in a problem that the amount of coarse particlesto be a cause of formation of polishing scratches is increased. Such aproblem is caused by, for example, a high roasting temperature or a toolong roasting duration. Further, if F2/F1 is lower than 0.7, the costfor the waste gas treatment is problematically increased. Incidentally,unless a fluorine component is supplied, F2/F1 does not exceed 1 and itis generally lower than 1. Further, F2/F1 is preferably 0.75 or higherbut 0.95 or lower. If within the range, roasting can further stably becarried out.

In this production method, roasting is carried out while adjusting theroasting temperature, the roasting duration, the gas circulation statein a roasting furnace, and the supply speed to the roasting so as tokeep F2/F1 in a range 0.7 or higher but 1 or lower. Hence, the contentof fluorine is easily controlled and the polishing properties and thepolishing evaluation can properly be adjusted. Consequently, acerium-based abrasive can be produced with excellent polishingproperties such as scarcity of scratch formation at the time ofpolishing, capability of providing smoothness of the polished face of anobject obtained after polishing, and the like. In such a manner, theproduction method of the invention has a characteristic that anattention is paid only to the fluorine content and its ratio alone iscontrolled but the roasting temperature, the roasting duration, theroasting quantity per time and other variety of operation conditions arenot required to be separately controlled. Such a method is extremelysimple and easy to control the roasting step and preferable.

Further, investigation has been made regarding the control of the colorof a cerium-based abrasive by the production method of the cerium-basedabrasive. The color of the cerium-based abrasive is affected by thecontent of fluorine, the oxidation degree, the specific surface areaexpressing the particle size and the like. Consequently, controlling thecolor of the raw materials of the cerium-based abrasive to be a desiredone by controlling those factors makes it possible to produce thecerium-based abrasive with excellent polishing properties. However,conventionally, an easy method for controlling the color, especially, amethod for easily controlling the color in the roasting step, has notbeen made available. For example, as described above, although the colorcontrol of the cerium-based abrasive is made possible by adjusting theroasting temperature, the roasting duration, the gas circulation statein a roasting furnace, and the supply speed to the roasting furnace, itis not necessarily always precise to obtain a desired color of thecerium-based abrasive.

As a result, the production method of the cerium-based abrasive of theinvention has been found suitable for controlling the color of thecerium-based abrasive. This production method, as described above,employs an extremely simple and easy means as the roasting step and itis therefore supposed to possible to relatively easily adjust the colorof the cerium-based abrasive.

For example, if F2/F1 is controlled to be in a range 0.7 or higher but 1or lower by the roasting step in the production method of thecerium-based abrasive according to the invention, the cerium-basedabrasive can be produced with a color having an L* value 65 or higherbut 90 or lower in the case of defining the color with the L*a*b* colorsystem. The cerium-based abrasive produced in such a manner, asdescribed above, has advantages that it has a sufficiently highpolishing speed, scarcely causes polishing scratches, and is capable ofproviding a polished surface with excellent smoothness and a highpolishing precision accompanied with little remaining abrasive.

As raw materials for the cerium-based abrasive to be employed for theproduction method of the cerium-based abrasive of the invention, thosepreferable have the content of praseodymium oxide 1% by weight or higherand 10% by weight or lower in TREO and the fluorine content F1 inrelation to TREO before roasting 0.5% by weight or higher but 14.3% byweight or lower.

The fluorine content in relation to TREO of the cerium-based abrasivecan be assumed to be F2 (the fluorine content in relation to TREO afterroasting). Consequently, F2 is to be preferable, for example, 0.5% byweight or higher but 10% by weight or lower. In this case, taking theproduction condition, “F2/F1 is 0.7 or higher and 1 or lower”, asdescribed above into consideration, it is preferable for F1 to be 0.5%by weight or higher but 14.3% by weight or lower. Further, in order tokeep F2 more reliably be 0.5% by weight or higher but 10% by weight orlower, F1 is preferable to be 0.7% by weight or higher but 10% by weightor lower. The reasons for the preference for such ranges are asdescribed above. Further, in the production of the cerium-basedabrasive, in th case of a high fluorine content, waste gas treatment atthe time of roasting is required and a problem of corrosion of furnacematerials inconveniently takes place. In consideration of such a pointof view, F1 is preferable to be 10% by weight or lower.

Controlling F2/F1 in the roasting step and at the same time controllingthe content of praseodymium oxide in TREO of raw materials and thefluorine content F1 in relation to TREO before roasting make it easy toproduce the abrasive having not only an L* value in a range 65 or higherand 90 or lower but also an a* value 0 or higher and 15 or lower and ab* value 10 or higher and 30 or lower.

Further, in the case the fluorine content in relation to TREO in the rawmaterials for the cerium-based abrasive is lower than 0.5% by weight, oreven if it is 0.5% by weight or higher, in the case the fluorine contentin relation to TREO is wanted to be increased, the treatment withfluorine as described above is carried out to control F1 0.5% by weightor higher but 14.3% by weight or lower. Incidentally, the content ofTREO in the cerium-based abrasive to be produced, the contents of ceriumoxide and praseodymium oxide in TREO, and further the fluorine contentare as described above.

The contents of cerium oxide and rare earth oxides such as praseodymiumoxide in TREO can be assumed to be unchanged between those in the rawmaterials for the cerium-based abrasive and the cerium-based abrasive.Consequently, those having the contents of cerium oxide and praseodymiumoxide in TREO within the ranges of the values defined to be preferablein the cerium-based abrasive are preferable to be used as raw materialsfor the cerium-based abrasive. Incidentally, the content of TREO in theraw materials sometimes becomes lower than 80% by weight depending onthe contents of water and carbonate residue in the raw materials,however usually since impurities are slight, the content of TREO reaches80% by weight or higher after roasting and therefore it does not cause aproblem.

Further, in the roasting step, it is preferable that the roastingtemperature is 600° C. or higher but 1,200° C. or lower and the roastingduration is 1 hour or longer but 60 hours or shorter.

In the roasting, cerium-containing particulate rare earth elements asraw materials are subjected to particle growth to a size enabling properpolishing speed. As the roasting means, for example, an electricfurnace, a rotary kiln, and the like can be employed. The ambientatmosphere is preferable to be oxidative atmosphere and, for example,the atmospheric air can be used as the ambient atmosphere. Generally, ifthe roasting temperature is high, abnormal particle growth easily takesplace and the amount of coarse particles in the abrasive is easy to beincreased, however it has been found as a result of the investigationthat praseodymium compounds including praseodymium oxide suppress theabnormal particle growth and at the same time promotes even roasting ofthe raw materials to be effective to suppress the amount of the coarseparticles. Consequently, if raw materials containing praseodymiumcompounds including praseodymium oxide are used, roasting can be carriedout at a higher temperature. If roasting is carried out at a hightemperature, sintering is easy to be promoted, so that it is especiallyeffective in the case an abrasive with a large average particle size isobtained and in the case an abrasive with a large average particle sizeand a sufficiently high polishing speed. Yet the content of praseodymiumcompounds including praseodymium oxide in the raw materials is notparticularly restricted, the proper content is to give the content ofpraseodymium oxide within a range 1% by weight or higher but 10% byweight or lower in TREO in the abrasive. Incidentally, the color of thecerium-based abrasive is affected by the roasting temperature, the abovedescribed temperature range is preferable from this point of view.

MODES FOR CARRYING OUT THE INVENTION

Hereinafter, practical embodiments of the invention will be described.

At first, raw material slurries were obtained by pulverizing rawmaterials for cerium-based abrasives by a wet type ball mill.Incidentally, as the raw materials, bastnasite concentrate, rare earthoxides, and cerium oxide were used. After that, no fluorine treatmentwas carried out for Example 1, Example 2, and Comparative Example 2. Onthe other hand, fluorine treatment was carried out respectively in orderto increase the fluorine content for Example 3 and Comparative Example1, in order to add fluorine to the rare earth oxides as raw materialsfor Examples 4 to 6 and Comparative Example 3, and in order to addfluorine to cerium oxide for Comparative Example 4. After that, theslurries were filtered and dried at 120° C to obtain powders. Therespective powders obtained after drying were roasting in a range of 800to 1,100° C. for 3 hours. Then, the powders obtained after roasting werepulverized and classified in order to decrease coarse particles with asize of 10 μm or larger as much as possible and thus cerium-basedabrasives were obtained.

The contents of cerium oxide (CeO₂) and praseodymium oxide (Pr₆O₁₁)before roasting, the fluoride content (<F1>) before roasting, thefluoride content (<F2>) after roasting, the content of TREO afterroasting, and the yield (<F2/F1>) of fluoride by roasting were measured.The measurement results were shown in Table 1.

The fluorine content is a value measured by an alkaline melting-warmwater extraction-fluoride ion electrode method. That is a measurementmethod as follows. At first, an abrasive or raw materials of theabrasive as specimens were melted with an alkaline melting agent,subjected to extraction with warm water after being cooled spontaneouslyand made to be constant in volume. A proper amount of the resultingspecimen was sampled, mixed with a buffer solution and then adjusted tohave pH about 5.3 and made to be constant in volume to obtain a samplesolution. Also, standard solutions were prepared. The operation forobtaining the standard solutions was same as that for obtaining thesample solution except that no specimen was used and a fluoride ionstandard solution was added after the sampling to adjust the fluorideion content to be a desired value. Several kinds of standard solutionswere obtained by changing the fluoride ion concentration. Regarding thestandard solutions and the sample solutions obtained in such a manner,the respective fluoride ion contents were measured using an ion meterequipped with a fluoride ion electrode. More practically, the fluorideion concentrations of the sample solutions were obtained based on acalibration curve obtained by measurement for the standard solutions andthe obtained fluoride ion concentrations were converted into thefluorine contents of specimens. Further, the obtained fluorine contentswere divided with the TREO contents of specimens described below toobtain the fluorine content in relation to TREO. Incidentally, thefluorine contents before roasting were measured by drying specimensbefore roasting at 120° C. for 2 hours and subjecting the resultingspecimens to the same operation as that carried out for the abrasivesafter roasting.

The TREO contents were measured as follows. At first, specimens weredecomposed using perhydrochloric acid and hydrogen peroxide and thenoxalic acid was added to the resulting solutions and pH of the solutionswas kept about 1.5 to obtain precipitates. The precipitates werefiltered and obtained precipitates were roasted at 1,000° C. The weightsof the roasted materials were measured and the TREO content wascalculated based on the weight of the roasted materials in relation tothe weights of the specimens.

Cerium oxide and praseodymium oxide contents were measured using analkaline melting agent and ICP emission spectrophotometry. At first,specimens were dissolved in an acid or alkaline melted and then therespective specimens were sampled in a proper amount and made to beconstant in volume to obtain samples. Also, several kinds of standardsamples with changed concentrations of cerium or the like were prepared.The contents of cerium or the like of the samples were quantitativelymeasured based on the calibration curve showing the concentrations ofcerium or the like obtained by measurement for the standard samples, andwere converted into the cerium oxide and praseodymium oxide contents.The cerium oxide and praseodymium oxide contents in TREO were calculatedfrom the cerium oxide and praseodymium oxide contents in the respectivesamples and the contents of TREO in the samples.

TABLE 1 CHEMICAL ANALYSIS RESULTS AND YIELD OF FLUORINE Chemicalanalysis before Roasting Chemical analysis after roasting step roastingPr₆O₁₁/ F/TREO Roasting F/TREO Yield of CeO₂/TREO TREO <F1> temperatureTREO <F2> fluorine Raw material (wt %) (wt %) (wt %) (° C.) (wt %) (wt%) <F2/F1> Example 1 bastnasite 50 3 8 800 83 6.7 0.84 Example 2bastnasite 50 3 8 900 85 6.0 0.75 Example 3 bastnasite 50 3 10 1000 897.1 0.71 Example 4 rare earth 60 5 8 850 90 7.1 0.89 oxides Example 5rare earth 60 5 8 900 92 6.8 0.85 oxides Example 6 rare earth 60 5 81000 93 6.1 0.76 oxides Comparative bastnasite 50 3 15 1000 84 7.3 0.49Example 1 Comparative rare earth 60 5 <0.1 900 97 <0.1 —¹⁾ Example 2oxides Comparative rare earth 60 5 8 1100 94 4.3 0.54 Example 3 oxidesComparative cerium oxide 99 0 3 800 99 <0.1 <0.03   Example 4 ¹⁾Nosignificant numerals were calculable.

As it is made clear from Table 1, in Examples 1 to 6, the yield offluorine (F2/F1) was 0.7 or higher. It was supposedly owing to theeffect attributed that the contents of cerium oxide, praseodymium oxideand fluorine were in respectively prescribed ranges. On the other hand,in Comparative Example 1, the yield was low. It was supposedly owing toexcess fluorine relative to praseodymium oxide. In Comparative Example3, the yield of fluorine was low. It was supposedly attributed to thehigh roasting temperature. In Comparative Example 4, almost all fluorinewas dispersed and dissipation of fluorine could not be prevented byroasting. It was supposedly attributed to no content of praseodymiumoxide.

Further, the cerium-based abrasives obtained in the respective examplesand Comparative Examples were subjected to color measurement (by a colorand color difference meter: CR-300, manufactured by Minolta Co., Ltd.)and the specific surface area measurement (by a BET specific surfacearea measurement apparatus using nitrogen gas: Multisorp, manufacturedby Uasa Ionics Co.). Further, a polishing test was carried out tomeasure the polishing properties of finally obtained cerium-basedabrasives. The measurement results and the results of the polishing testwere shown in Table 2. Incidentally, although the color measurement wascarried out after pulverization after the roasting, in other words,before the classifying step, the color of each abrasive product wasfound almost the same before and after the classifying step, so that thecolors of the cerium-based abrasives wer shown in Table 2.

Incidentally, in the polishing test, an Osker type polishing machine(HSP-21 model, manufactured by Taito Seiki Co., Ltd.) was employed. Forthe polishing test, cerium-based abrasive slurries having the abrasiveconcentration of 10% by weight and prepared by dispersing the powdertype cerium-based abrasives in water were used. The object to bepolished was a glass material for a flat panel with 65 mmφ and apolishing pad made of a polyurethane was used. The polishing conditionswere as follows: the rotation speed of the glass material was 1,700 rpm;the pushing pressure of the pad was 98 kPa (1 kg/cm²), and the polishingduration was 10 minutes.

The polishing values in Table 2 were the values based on the polishingamounts calculated by measuring the weight of the glass material beforeand after polishing. They were expressed as relative values in the casethe value of Example 1 was set to be 100.

The polishing scratches were evaluated as follows. At first light of ahalogen lamp with 300,000 lux light source was radiated to the surfaceof the glass for a flat panel after polishing and the number and thesize of the scratches were judged by a reflection method. Then,corresponding to the number and the size of the scratches, the resultswere numerary expressed by a minus-point system on the basis of 100marks. In Table 2, “E” showed to be 95 or higher and 100 or lower marksand extremely suitable for precision polishing; “G” showed to be 90 orhigher but lower than 95 marks and suitable for precision polishing; “F”showed to be or higher 80 and lower than 90 marks and usable for generalpolishing; and “P” showed to be lower than 80 and unsuitable as anabrasive, respectively.

The evaluation of the remaining abrasives was carried out as follows. Atfirst, the glass after polishing was washed in pure water using anultrasonic washing apparatus and dried in dust-free state. After that,the glass surface was observed by an optical microscope and theexistence of the residual abrasives adhering to the glass surface wasobserved. In Table 2, “E” showed the residual abrasives were scarcelyobserved to prove extreme suitability as an abrasive; “G” showed theresidual abrasives were slightly observed to prove suitability as anabrasive; and “P” showed many residual abrasives were observed to proveunsuitability as an abrasive, respectively.

TABLE 2 COLOR, THE SPECIFIC SURFACE AREA, AND POLISHING PROPERTIES OFCERIUM-BASED ABRASIVE Physical measurement after roasting Evaluation ofpolishing Measured color of powder Specific surface Polishing PolishingResidual L* value a* value b* value area (m²/g) value scratch¹⁾abrasive²⁾ Example 1 85.52 4.86 17.64 7.9 100 E G Example 2 77.87 5.9420.78 2.5 180 E E Example 3 74.53 8.76 22.45 1.6 330 G E Example 4 75.4811.27 19.84 6.1 120 E G Example 5 74.12 12.55 21.29 3.5 155 E E Example6 72.23 14.90 24.12 2.9 170 E E Comparative Example 1 64.43 16.85 18.871.0 400 P E Comparative Example 2 49.74 11.05 9.36 13.2 25 P PComparative Example 3 60.19 19.41 19.28 0.7 450 P E Comparative Example4 94.53 −1.95 2.40 9.5 75 G P *¹⁾The reference characters of thepolishing scratch respectively denote as follows: “E”: extremelysuitable for precision polishing if the mark is 95 or higher and 100 orlower. “G”: suitable for precision polishing if the mark is or higher 90and lower than 95. “F”: usable for general polishing if the mark is orhigher 80 and lower than 90. “P”: unsuitable as an abrasive if the markis lower than 80. ²⁾The reference characters of the residual abrasiverespectively denote as follows: “E”: extremely suitable as an abrasive.“G”: suitable as an abrasive. “P”: unsuitable as an abrasive.

As shown in Table 2, in Examples 1 to 6, the L* values in the L*a*b*color system were all in a range 65 or higher but 90 or lower: the a*values were all in a range 0 or higher but 15 or lower: b* values all ina range 10 or higher but 30 or lower: and the specific surface area was1 to 30 m²/g, proving any of the polishing values, the polishingscratches, and the residual abrasives, which were polishing properties,to be excellent.

However, in Comparative Example 1, fluorine was found existingexcessively and the specific surface area was small and the polishingscratches were many. The b* value was 18.87, whereas the L* value waslower than 65 and the a* value was higher than 15.

In Comparative Example 2, the specific surface area was high and thepolishing value was extremely low and the residual abrasive was high inquantity. It was supposedly attributed to scarce existence of fluorine.The L* value was 49.74, lower than 65 and the a* value was 1.05, lowerthan 15, and further the b* value was 9.36, lower than 10.

In Comparative Example 3, many polishing scratches were observed. The b*value was 19.28, whereas the L* value was 60.19, lower than 65 and thea* value was 19.41, higher than 15.

In Comparative Example 4, the residual abrasive was high in quantity.The L* value was 94.53, higher than 90. It was supposedly attributed tono existence of praseodymium oxide. Further, the a* value was −1.95 withthe inverse sign showing the color change toward green and also the b*value was 2.40, extremely small as compared with 10.

Industrial Applicability

The invention can provide a cerium-based abrasive excellent in thepolishing properties; a sufficiently high polishing speed and scarceprobability of causing polishing scratches and leaving little remainingabrasive. Further, the invention provides a production method of such acerium-based abrasive. The abrasive provided can be used for surfacepolishing with a high precision required in production of glasssubstrates for optical disks and magnetic disks and the like.

1. An abrasive containing cerium oxide as a main component, wherein theabrasive contains fluorine and has an L* value of 65-90 and wherein thecolor of the abrasive is expressed by an L*a*b* color system.
 2. Theaccording to claim 1, wherein the a* value in the L*a*b* color system is0-15.
 3. The abrasive according to claim 1, wherein the b* value in theL*a*b* color system is 10-30.
 4. The abrasive according to claim 1,wherein the content of total rare earth oxides (TREO) in the abrasive is80-99% by weight and the content of fluorine relative to TREO is 0.5-10%by weight.
 5. The according to claim 4, further comprising praseodymiumoxide in the amount of 1-10% by weight based on the total weight % ofTREO.
 6. The according to claim 1, in the form of particles having aspecific surface area of 1-30 m₂/g.
 7. A method for producing anabrasive containing cerium oxide as a main component and fluorine,comprising roasting raw materials for the abrasive, wherein a ratioF2/F1 is 0.7-1, and wherein F1 is the content of fluorine in the rawmaterials in relation to TREO before roasting F1 and F2 is the contentof fluorine in the abrasive in relation to TREO after roasting.
 8. Themethod according to claim 7, wherein raw materials for the abrasivefurther comprise praseodymium oxide in the amount of 1-10% by weightbased on the total weight % of TREO and wherein F1 is 0.5-14.3% byweight.
 9. The method according to claim 8, wherein the roasting is at aroasting temperature of 600-1,200° C. and a roasting duration of 1-60hours.
 10. A method for improving the polishing properties of anabrasive containing cerium oxide as a main component, the methodcomprising adding fluorine to the abrasive in an amount such that theabrasive has an L* value of 65-90 when the color of the abrasive isexpressed by an L*a*b* color system.